Brewing with Potatoes: Techniques

Both corn and rice are used as starchy adjuncts by brewers worldwide. These adjuncts boost the strength of a beer without increasing its body. Corn and rice also dilute the protein content of wort. As adventurous homebrewers, there is another common starchy food we can use as an adjunct — potatoes.

Types of Potatoes

The common potato comes from the potato plant (Solanum tuberosum), a member of the nightshade (Solanaceae) family. The nightshade family also includes tomatoes, tobacco and peppers. The edible portion of the potato plant is the tuber, a modified underground stem. There are many varieties of potatoes found on supermarket shelves and they can be grouped into two functional categories, waxy or mealy (or starchy). Mealy varieties — such as Russet, Yukon Gold or baking-type potatoes — can easily be used in homebrewing. Waxy varieties — such as Chef's potatoes or red potatoes — may be usable, but I don't have any experience with them.

Potato Composition

At harvest, potatoes contain about 78% water, 18% starch and 2–3% protein. The remainder of the potato is a variety of compounds including fiber and a tiny amount (around 0.1%) of fat. Potatoes also contain relatively high levels of vitamin C.

The starch in potatoes is arranged in larger granules than those found in most plants, including barley. Potato starch is about 20% amylose and 80% amylopectin. In comparison, barley starch is 20–25% amylose and 75–80% amylopectin. Potato starch gelatinizes at relatively low temperatures, around 130 °F (54 °C) for most mealy varieties. As such, the starch is accessible to the saccharification enzymes at typical mash temperatures — 148–158 °F (64–70 °C).

Potatoes can be dried and extruded through rollers to produce potato flakes. Potato flakes are usually made from Russet potatoes and contain 5–7.5% moisture, 60–75% starch and 7–9% protein. Many potato flakes, such as those designed to make instant mashed potatoes, are salted or otherwise seasoned. When using potato flakes in brewing, be sure to check the ingredient label and select only unseasoned potato flakes. Flakes will likely contain sodium bisulfite, a chemical added to prevent non-enzymatic browning of the flakes during storage. They may also include an emulsifier, often a monoglyceride or diglyceride. These preservatives and processing agents will be diluted in your wort and should not interfere with your brewing.

The percentage of protein in potato flakes is on par with the percentage found in malted barley. So, you don't need to worry about the potatoes contributing to protein haze in your beer. However, they contain more protein than flaked maize or rice, so you can't use potatoes to dilute the protein content of your wort when using high-protein malts.

All-grain brewers can use either raw potatoes or unseasoned potato flakes in brewing. Using potato flakes is simpler, but I've always used unprocessed potatoes in my beers to avoid the processing agents in potato flakes, even though I'm almost positive that they would be harmless. Homebrewers used to extract-with-grains procedures can easily make partial mash beers using potatoes.

Don't look at potato beer as a way to get rid of potatoes that have sprouted — potato sprouts contain toxins and should therefore be discarded. If your potatoes aren't fit for cooking, they aren't fit for brewing.

Brewing with Potato Flakes

Potato flakes can be stirred into the mash just as flaked maize would be. Whenever I mash any starchy adjunct, I stir the mash a few times during the saccharification rest. Other than that, however, just follow your normal mashing procedures and finish brewing as you normally would.

For recipe calculations, expect a potential extract of about 35 points per pound per gallon. (In other words, if a pound of potato flakes are used to make a gallon of wort, that wort would register a specific gravity of 1.035.) Remember that this number will be modified by your extract efficiency. For example, if you get 65% extract efficiency, potato flakes will yield about 23 points per pound per gallon.

As when using any other starchy adjunct, don't use over 40% potato flakes (by weight) when using six-row malts. When using 2-row malts, 30% potato flakes is a good upper limit.

Using Unprocessed Potatoes

For all my "spud-braus," I've simply used plain, unprocessed potatoes. To prepare the potatoes for brewing, I peel them and cut them into small (1 inch/2.5 cm) cubes. I then boil the cubes in enough water to cover them. After 15 minutes or so, I drain the water and mash the potatoes with a potato masher. ("Mash" here means crush or whip, not the brewing term.) You could also whip the potatoes with an electric beater. At this point, I essentially have mashed potatoes, although the usual addition of salt or other seasonings have been omitted. I'll call them whipped potatoes from here on out, to avoid confusion over the word "mash."

When brewing with potatoes, I add the foundation water (the water under the false bottom) to my mash tun then fill the vessel with the crushed grains. I then stir in the whipped potatoes. Unlike when adding flaked maize or other dried starchy adjuncts, adding freshly prepared whipped potatoes to the mash adds heat and a small amount of water to the mash. However, unless you are going titanic with the tubers, this doesn't amount to a huge difference at mash in. Expect to add slightly less strike water and be prepared to add a small amount of room temperature water as you approach your desired mash thickness due to the added heat from the potatoes. If you dislike "winging it" when brewing, simply stir your whipped potatoes into your strike water prior to mashing in. That way you can heat this "soup" to your normal strike water temperature when mashing in.

Once I've mashed in, I brew the beer as I normally would except for stirring the mash a few extra times. I've never encountered any problems with the mash sticking during the runoff, even though the potatoes leave behind a small amount of solids in the grain bed.

For recipe calculations, begin with the fact that potatoes have 22% dry weight and of that dry weight, 75% is starch. Assuming that this starch is completely reduced to sugar means that raw potatoes yield about a potential extract of 7.6 points per pound per gallon. (As with the potential extract of malted barley, this number will be modified by your extract efficiency.) Looked at another way, 5.0 lbs. (2.2 kg) of potatoes, with a dry weight of 1.1 lbs. (0.5 kg), have an equivalent potential extract as 1.0 lb. (0.45 kg) of 2-row pale malt.

Effects of Potato Usage

Potatoes do not add any potato flavor to your beer. They simply add fermentable sugar to the wort. Like corn or rice — or the addition of sugars to the kettle — potatoes dry out a beer. See the recipe for Left of Lefse Extra-Dry Stout (January-February 2004 BYO) for a recipe using Russet or Yukon Gold potatoes to produce a nice dry session beer. Potatoes can also be used to make thirst-quenching lawnmower beers. In fact, during World War II, the Lucky Lager brewery used potatoes as an adjunct because of the rationing of corn. See the recipe on page 50 for an eminently quaffable lawnmower beer.

If you want to use potatoes, but don't want your beer to turn out dry, you have a couple options. Mashing at higher temperatures (and in thicker consistencies) yields a less fermentable wort and correspondingly fuller-bodied beers. Likewise, adding more specialty malts will boost the body of a beer. Either or both of these methods can counteract the drying effect of potatoes if a fuller bodied beer is desired.

Other "Potatoes"

There are many interesting alternatives to the common potato that can potentially be used as starchy adjuncts when brewing.

Sweet potatoes (Ipomoea batatus) are familiar vegetables to most people. But these yellow or orange vegetables, commonly served at Thanksgiving dinner, are not true potatoes. They are actually members of the morning glory (Convolvulaceae) family. The edible part of the sweet potato plant — the "potato" — is a storage root, not a tuber. That's why sweet potatoes often have spindly roots connected to them.

Compared to potatoes, sweet potatoes have more starch and less protein. They also contain 3–6% sugar. I've used the orange variety of sweet potatoes and they add a very nice orange hue to beer. They don't, however, add any sweet potato flavor. I suspect that most of the flavor is associated with the sugars, which just ferments away. I've made a sweet potato ESB by substituting 5.0 lbs. (2.3 kg) of sweet potatoes for 1.0 lb. (0.45 kg) of pale malt in a basic ESB recipe. The substitution yields an ESB that tastes normal, but has an interesting orange hue.

It's possible that roasting the sweet potatoes will develop that flavor to the point it shows up in the finished beer, but I've yet to try this. You may also need to add some sweetness, perhaps by adding lactose, to make the sweet potato flavor recognizable.

Yams (from many species of the genus Dioscorea in the family Dioscoreaceae) also appear in many supermarkets, although it's not uncommon for vegetables labeled as yams to really be sweet potatoes. Yams are more closely related to lilies than they are to either potatoes or sweet potatoes. These variably colored tubers contain about 20% starch and are sweeter than sweet potatoes. I've never used them in brewing, but can't see any reason why they wouldn't work just like sweet potatoes.

Cassava (Manihot esculenta) is a starchy storage root from which tapioca pudding is made. Cassava is grown in the tropics, where cereals or potatoes do not grow well. As with yams, I haven't tried it, so maybe you can be the first homebrewer on your block to give it a whirl.

Collect 5 gallons (19 L) of wort, adding first wort hops after collecting the first gallon (3.8 L) of wort. Add 1 gallon (3.8 L) water and boil wort for 90 minutes, adding hops at times indicated in recipe. Add Irish moss and yeast nutrients with 15 minutes left in boil. After the boil, the specific gravity of the wort should be 1.053.

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